Port 500: IKE (IPsec VPN)
UDP port 500 is IKE, which sets up the encryption keys for IPsec VPN connections.
What runs on port 500
IPsec VPNs negotiate keys with the Internet Key Exchange protocol on UDP 500 before any traffic flows. It is used by site-to-site VPNs between offices and by client VPNs such as IKEv2 on iPhone, Mac and Windows. When a device is behind NAT, the session moves to UDP 4500.
On a home network
Routers with a built-in IPsec or IKEv2 VPN server listen on 500. Xbox consoles also list UDP 500 among the ports they use to connect out. Phones and laptops using an IKEv2 VPN connect out to it.
If port 500 is reachable from the internet
A VPN endpoint is meant to be reachable from the internet, so 500 being open on a router that runs your VPN is normal. What matters is that the VPN firmware is patched and uses modern settings, since VPN gateways are a favourite target for attackers once flaws are published. If you do not run an IPsec VPN, nothing should answer on 500.
How to check whether it is open on your network
Check your router's VPN server settings to see whether IPsec or IKEv2 is enabled. An outside scan of UDP 500 on your public address confirms whether anything answers.
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